The present disclosure relates, in various exemplary embodiments, to reimageable recording medium comprising a protective layer. More specifically, the present disclosure relates to a reimageable recording medium comprising a protective layer that is capable of exhibiting and switching between i) a UV transparent state to allow for forming an image on an underlying display layer, and ii) a UV absorbing state to protect unimaged areas of the recording medium from exposure to incident UV light under viewing conditions that may discolor the unimaged areas.
One method for providing a reimageable recording medium is to provide a recording medium coated with a photochromic material. Photochromic materials change from a colorless to a colored state when exposed to ultraviolet light. Co-pending application Ser. Nos. 10/835,518 and 10/834,722 (the contents of which are totally incorporated herein by reference) are respectively directed to a reimageable recording medium comprising a photochromic material and a method for forming an image using such a reimageable recording medium. Some examples of known photochromic materials include spiropyrans and spiroxazines. Upon exposure to ultraviolet light, the closed ring structures, which exists in a colorless state, open and exhibit a colored state. The formulas below depict the closed and open states of a spiropyran and spiroxazine, respectively.

Readable or viewable images are formed by exposing selected areas of a medium comprising a photochromic material to UV light, typically having a wavelength of 365 nm, to cause the photochromic material to change from the colorless state to the colored state. The printed information or image is viewable for a limited period of time, and the image self-erases as the photochromic material changes back to the colorless form. This leaves a blank medium (e.g., a document) that is ready to be reimaged as desired with new information.
The visual quality, and hence, readability of the document depends on the resolution of the imaged areas. The resolution depends on the contrast ratio between the colored (imaged) areas of the document and the colorless (unimaged) areas of the document. The unimaged areas are typically white (depending on the type of substrate used). Unimaged areas, however, are sensitive to the UV component of the light used to read the document (e.g., UV-VIS, room-light such as from a light bulb, sun-light, and the like). Therefore, unimaged areas become slightly colored over time and reduce the contrast between the white and colored states and therefore also reduces the readability of the document.
Attempts to solve this problem have been made by creating a band-pass window for the incident light capable of isomerising (i.e., inducing coloration) the material centered around 365 nm. The stability of such transient documents has been significantly improved when compared to unprotected documents. The unimaged areas, however, are still sensitive to the UV component of visible light centered at 365 nm.
Co-pending application U.S. patent application Ser. No. 11/139,890, now allowed, the entire disclosure of which is incorporated herein by reference, discloses photochromic protective layers for a selectively imageable member. The protective layers comprise a material that is capable of switching between a UV absorbing state and a UV transparent state. The UV transparent state is obtained by illumination with high intensity UV light. In this state, UV light is able to be transmitted through the protective layer to an underlying imaging layer comprising a photochromic material that is converted to its colored state to form an image. After a period of time, the protective material reverts back to the UV absorbing state and prevents incident UV light from coloring the unimaged areas.
There is still a need to provide protective materials that are sensitive to longer wavelengths (i.e., 400 nm). Secondly, there is also a need to provide these materials in solution form that allows the materials to switch between the desired UV transparent and UV absorbing states. Further, and separate and apart from the above, there is a need to provide protecting materials in a microencapsulated form that can be applied as a coating on top of the imaging material.